CAMP DAVID, Maryland (CNN) -- Afghan President Hamid Karzai said Monday that the Taliban still endanger innocent people, but they pose no significant threat either to his government or to its institutions.

"It's a force that is defeated, frustrated, acting in cowardice by killing children going to school," Karzai told reporters during a joint news conference with President Bush. Karzai said he is working to root the Taliban out from their mountain hideouts.

For his part, Bush described the Taliban, who ruled Afghanistan and harbored al Qaeda before the September 11, 2001, attacks, as "brutal coldblooded killers" with "a vision of darkness."

Bush noted that speculation last spring that the Taliban would launch an offensive against Karzai's government was countered by a U.S., NATO and Afghan offensive. Bush said Afghanistan's 110,000 troops are being aided by 23,500 U.S. troops and 26,000 troops from other nations.

Since 2001, "we've committed more than $23 billion" to help rebuild Afghanistan, Bush said.

In addition, more than 640 coalition troops -- most of them Americans -- have been killed.

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During the news conference, the two leaders vowed to work together and with others to thwart terrorism of all kinds.

Karzai said he hopes that a jirga with Pakistan leaders -- scheduled to begin Thursday in Kabul, Afghanistan -- will help him determine who is supporting the Taliban. A loya jirga is Afghanistan's traditional assembly.

"I hope very much that this jirga will bring to us what we need, which I think it will," he said.

Asked whether he would act unilaterally if he had intelligence about al Qaeda leaders in Pakistan, or whether he would wait for permission from Pakistan's president, Gen. Pervez Musharraf, even if that were to mean missing an opportunity to attack, Bush did not respond directly.

"We're in constant communications with the Pakistan government," he said, adding, "I'm confident that, with real actionable intelligence, we will get the job done."

"He is as concerned as the Afghan people are," Karzai said. "I was very happy with that conversation."

Bush said he regrets the loss of innocent life, but he put the blame on the Taliban. "It is the Taliban who surround themselves with innocent life as human shields," he said. "The Taliban are the coldblooded killers; the Taliban are the murderers; the Taliban have no regard for human life."

Asked about Karzai's comments last week that Iran has proven to be a stabilizing force in Afghanistan, Bush didn't appear persuaded. "It is up to Iran to prove to the world that they are a stabilizing force as opposed to a destabilizing force," he said. "After all, its government proclaimed its desire to build nuclear weapons."

Iran has said its nuclear program is intended solely for peaceful purposes, but Bush accused the country of appearing "to be willing to thumb its nose at the international community."

Bush added, "I'm willing to listen, but from my perspective, the burden of proof is on the Iranian government to prove to us that they are a positive force."

He said the current leadership of the country, which he once described as being part of an "axis of evil," is "a big disappointment to the people of Iran."

And Bush vowed to "continue to work to isolate it. They are not a force for good, as far as we can see."

Karzai let the matter drop. Instead, he praised a U.S. health program that, he said, had saved the lives of 85,000 Afghan children.

Throughout the news conference, he was effusive in his gratitude to the United States, and promised to work to stem the trade in poppies from his country, which feed the habits of many U.S. drug users. "We are committed to fighting it," he said. But he added, "It will take time."

Karzai defended his decision to pardon a 14-year-old boy who had been recruited to be a suicide bomber. He said the boy had been brainwashed by terrorists. "The message should be clear to the rest of the world about the evil that we are fighting," he said.

The buying and selling of suicide bombers has become a trade, he said. "Merchants of death are around there; it's our job to get rid of them."

After the news conference, White House National Security Council spokesman Gordon Johndroe told reporters the two leaders agreed that there "should be no quid pro quo for hostages" -- that is no concessions to the Taliban in exchange for 21 South Korean hostages kidnapped in Afghanistan.

Two other South Korean abductees have been killed.

Bush is working with the governments of South Korea and Afghanistan "to the extent possible" in urging the hostages be released, Johndroe said.E-mail to a friend